The present invention relates to a recording material for the production of offset printing plates having a web- or plate-form support, a radiation-sensitive layer on the front of the support and a continuous layer on its back.
Recording materials for the production of offset printing plates (also known as xe2x80x9cpre-sensitized printing platesxe2x80x9d) are usually supplied in stacks of 20 units or more. Extended storage times, the action of pressure and/or elevated ambient temperatures frequently result in the plates adhering to one another. On removal of individual plates from the stack, scratches may then form on the front and/or back. The problem of undesired adhesion can be substantially eliminated with the aid of separating paper. The paper is particularly necessary in the case of recording materials having an aluminium support with an uncoated back. However, the separating paper results in new problems. The recording materials are frequently produced in in-line finishing plants, in which the plates are automatically cut to the desired size and packed. The separating paper is likewise inserted automatically. However, this step is relatively slow and susceptible to faults. In addition, the paper in some cases affects the radiation-sensitive layer and adversely changes its properties. This may result in discoloration of the layer, due to a change in the pH, a drop in its light sensitivity or rapid ageing. With surface-sealed papers, the interaction between paper and radiation-sensitive layer can be reduced; however, such papers are significantly more expensive. In relatively large print shops, the plate stacks provided with separating paper are generally processed in automatic plants, with the paper usually being blown out. This operation too is again relatively slow and susceptible to faults. In addition, the paper cannot be recycled and has to be disposed of.
The recording material described in JP-A 02/040657 manages without separating paper. A UV-cured layer produced from a photopolymerizable material is located on the back of its aluminium support. In addition to monomers, the composition used for the production of the back coating may also comprise photosensitizers, binders, fillers, inhibitors for preventing thermally induced polymerization of the monomers and other additives.
JP-A 06/202312 discloses a recording material for the production of offset printing plates whose aluminium support is likewise coated on the back with an organic polymer, such as polyethylene, polypropylene, polybutadiene, polyester, polycarbonate, polyvinyl acetal, polyvinyl chloride, polystyrene or a methacrylate resin. The back coating reduces attack by the aqueous-alkaline developer on the aluminium support. The light-sensitive layer in this recording material comprises from 1 to 10% by weight of a compound which is insoluble in the developer.
A recording material having an anodized aluminium support, a photopolymerizable layer on the aluminium oxide layer produced by anodization, and a back coating having a thickness of from 0.1 to 8.0 xcexcm is disclosed in JP-A 09/265176. This back coating consists of a saturated copolymerized polyester resin, a phenoxy resin, a poly-vinyl acetal or a vinylidene chloride copolymer, each of which has a glass transition temperature Tg of 20xc2x0 C. or above. This is intended to prevent scratching of the plates during transport in the stack and delamination of the radiation-sensitive layer due to excessive adhesion to the back of the overlying plate.
A recording material for the production of offset printing plates which can be stacked without separating paper is also described in EP-A 528 395. It comprises a support (made of aluminium), a layer of an organic polymeric material having a glass transition temperature of not less than 20xc2x0 C. with a thickness of from 0.01 to 8.0 xcexcm on the back of the support, and a light-sensitive layer on the front of the support. A discontinuous matting layer consisting of particles having a mean diameter of not greater than 100 xcexcm and a mean height of not greater than 10 xcexcm is in turn located on the light-sensitive layer. However, matting layers, in particular those comprising a material having a low glass transition temperature, tend to stick to the back of the overlying plate in the stack. This may cause relatively large areas of the radiation-sensitive layer to be delaminated, meaning that the recording material can then no longer be used further.
A recording material having a matting layer on the radiation-sensitive layer and a further, likewise discontinuous matting layer on the back of the support material is disclosed in EP-A 883 028. The support material is generally an aluminium web having a length of up to several thousand meters and a thickness of about 0.2 mm which is generally rolled up again after the coating (xe2x80x9ccoil-to-coil processxe2x80x9d). A higher pressure prevails in the interior of the roll than in the outer regions. This results in the matting layer located on the front being damaged to a non-uniformly great extent. A recording material having a greatly damaged matting layer requires longer evacuation times later in the contact copying frame. This problem is substantially solved by the additional matting layer on the back.
EP-A 490 515 relates to a pre-sensitized printing plate which, after imagewise exposure, is developed using an aqueous alkali metal silicate solution. In order to prevent the developer from dissolving aluminium out of the back of the plate, this is provided with an organic polymeric coating which is insoluble in the developer. The coating comprises polymers such as polyethylene, polypropylene, polybutene, polybutadiene, polyamide, polyurethane, polyurea, polyimide, polysiloxane, polycarbonate epoxy resins, polyvinyl chloride, polyvinylidene chloride or polystyrene. It may also comprise a thermally or photochemically curing component.
DE-A 199 08 529 describes a recording material having a support which has on the back a layer comprising an organic polymeric material having a glass transition temperature of 45xc2x0 C. or above, and a pigmented light-sensitive layer located on the front of the support.
DE-A 100 29 157 relates to a recording material for the production of offset printing plates which has a dimensionally stable support, a radiation-sensitive layer located on the front of the support, and a layer which comprises an organic polymeric material and is resistant to processing chemicals located on the back of the support, where the glass transition temperature of the organic polymeric material is 35xc2x0 C. or above, and the layer located on the back is pigmented.
The layer of recording materials known hitherto which is located on the back has high electrical surface resistance. It thus acts as an insulator, enabling electrostatic charges to build up. Since the back layer is advantageously applied before the image layer, it is necessary to dissipate any electrical charges present. The electrical charges may be residual charges (from the electrochemical treatment) and/or charges which form due to static charging during passage of the belt through the unit. Immediately after the electrochemical treatment, a radiation-sensitive mixture is applied to the pre-treated front of the support in the conventional in-line belt units. The mixture virtually always comprises readily volatile organic solvents which are explosive together with air. Static charges, which may result in ignition of a solvent vapour/air mixture, must therefore be avoided under all circumstances.
The object of the invention was to modify the known back coatings in such a way that they are electrically conductive. In addition, if the back coating comes into contact with a radiation-sensitive layer, it should not adversely change the latter. In particular, the back coating should not stick to the radiation-sensitive layer or adhere so strongly that damage occurs on removal of the individual plates from the stack.
The object is achieved by a back coating which is electrically conductive. To this end, it generally comprises conductive, organic polymers or conductive pigments. In some cases, amphiphilic low-molecular-weight compounds are also sufficient.
The present invention thus relates to a recording material for the production of offset printing plates having a web- or plate-form support, a radiation-sensitive layer on the front of the support and a continuous layer on its back, which is characterized in that the layer located on the back has a glass transition temperature Tg of 55xc2x0 C. or above and an electrical surface resistance of 1xc2x7106xcexa9 (determined in accordance with DIN 53482 using a spring tongue electrode at 23xc2x0 C. and 50% relative humidity) or less. The back coating may have a smooth or structured surface, depending on the nature of the electrically conductive component and the application method. The electrical conductivity is at least sufficiently large that the electrostatic charges which occur during electrochemical treatment of the support material can be dissipated.
As electrically conductive component, the back coating preferably comprises an electrically conductive polymer, such as polythiophene, polypyrrole, polyacetylene, polyaniline or poly(para-phenylene). The proportion of electrically conductive polymer(s) is generally from about 0.1 to 25% by weight, preferably from 0.5 to 10% by weight, in each case based on the total weight of the non-volatile constituents of the back coating.
Instead of or in addition to the electrically conductive polymer it is possible to use an electrically conductive pigment, such as a mica pigment coated with a metal (for example nickel) or metal oxide (for example antimony oxide or tin oxide), an aluminium pigment or carbon black. The proportion of electrically conductive pigments is generally from 1 to 25% by weight, preferably from 3 to 20% by weight, in each case based on the total weight of the non-volatile constituents of the back coating. Depending on the particle diameter, the pigments always produce a certain minimum roughness (Bekk value less than 500) of the surface of the back coating. By contrast, layers having a very smooth surface (Bekk value greater than 600) can also be produced with the electrically conductive polymers.
The electrical surface resistance of the back coating can be reduced to from about 1xc2x7106 to 1xc2x710xe2x88x922xcexa9, preferably from 1xc2x7104 to 1xc2x710xe2x88x922xcexa9, by means of the electrically conductive polymer and/or the electrically conductive pigment, depending on the nature and proportion of the additives.
For some cases, merely an amphiphilic, low-molecular-weight compound in the back coating is sufficient. This is, for example, an alkylsulfonate, an alkylarylsulfonate or -phosphonate, an ethoxylated alkylamine or a quaternary alkylamine. These compounds accumulate at the surface of the layer, where they result in the formation of an electrically conductive moisture film. However, these additives reduce the electrical surface resistance of the layer to only about 106xcexa9. This embodiment is therefore generally less preferred. If the pre-sensitized printing plates are stored in stacks, the amphiphilic, low-molecular-weight compound may in addition diffuse out of the back layer of one plate into the image layer of the adjacent plate and impair the copying properties of the latter.
Besides the said constituents which provide for electrical conductivity, the back coating generally comprises an organic, polymeric material which is virtually insoluble in water and aqueous-alkaline developers. By means of this material, the glass transition temperature of the back coating is set to the requisite value of 55xc2x0 C. or above. In one embodiment, the back coating is physically drying (i.e. not self-curing). Particularly suitable materials for a coating of this type are polyolefins (such as polyethylene, polypropylene, polybutylene, polybutadiene or polyisoprene), polyesters, polycarbonates, polyamides, polysiloxanes, polystyrene, homopolymers or copolymers of or with alkyl acrylate or alkyl methacrylate units (such as polymethyl methacrylate (PMMA) or styrene-methyl methacrylate copolymers), polyvinyl acetal, phenoxy resins (for example resins made from bisphenol A and epichlorohydrin), polyvinyl chloride (PVC) or polyvinylidene chloride (PVDC). If necessary, the layer may in addition comprise additives in secondary amounts. These include, for example, plasticizers, dyes, silicone compounds or surface-active agents. In the case of physically drying, non-self-curing back coatings, the organic polymeric material preferably has a glass transition temperature of 55xc2x0 C. or above. In a further embodiment, the back coating is self-curing. In this case, besides the organic polymeric materials, it also comprises monomeric or oligomeric compounds which polymerize, condense or crosslink on exposure to radiation, heat and/or oxidizing agents and thus effect curing of the layer. Particularly suitable for this purpose are addition-polymerizable acrylates or methacrylates, such as ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, 2-hydroxyethyl (meth)acrylate, trimethylolpropane mono-, di- or tri(meth)acrylate or pentaerythritol tri(meth)-acrylate. Also suitable are (meth)acrylamides, such as N-methyl-, N-ethyl-, N-propyl-, N-butyl- or N-isobutyl-(meth)acrylamide; furthermore allyl esters, such as allyl acetate; vinyl ethers, such as butyl vinyl ether, octyl vinyl ether, decyl vinyl ether, 2-methoxyethyl vinyl ether, diethylene glycol vinyl ether or benzyl vinyl ether; polyfunctional urethane acrylates which cure on exposure to UV radiation, and diisocyanate/diol mixtures which cure on exposure to heat to give polyurethanes.
In accordance with general practice, xe2x80x9c(meth)acrylatexe2x80x9d in the present application stands for xe2x80x9cacrylate and/or methacrylatexe2x80x9d. A corresponding meaning applies to xe2x80x9c(meth)acrylamidexe2x80x9d and other derivatives of acrylic or methacrylic acid.
As described, the curing of the back coating may also be induced by actinic radiation, i.e. the back coating is in this case light-sensitive. For distinction therefrom, the radiation-sensitive layer on the front of the support is referred to as xe2x80x9cimage layerxe2x80x9d, since only this is exposed imagewise and developed.
The back coating is produced with the aid of generally known application methods and devices. Particularly suitable is firstly roller application. Use can be made here of smooth or engraved rollers which effect direct or indirect (offset) application, co-rotating or counter-rotating application. Also suitable is application with the aid of a counterpressure roller or kiss coat. The electrically conductive back coating can have a smooth or structured surface.
The lacquer is initially located in a lacquer trough, from where it is applied to the support (which is preferably an aluminium web or an aluminium band having a thickness of from about 0.1 to 0.3 mm) by means of an uptake roller and an application roller. If necessary, one or more counter-rollers may also be employed here. Instead of an open lacquer trough, it is also possible to use a closed chamber doctor blade.
The structure here depends on the surface of the coating roller, the relative speeds of the rollers to one another and of the coating roller relative to the substrates. Depending on the magnitude of the speed difference between the application roller and the band, the lacquer structure created on the application roller is transferred to the band with little, if any, change. Of particular importance for this is of course the rheology of the lacquer, in particular its viscosity. The flow properties of the lacquer can be adjusted as required by warming the lacquer and/or the rollers and/or the band. In the case of transfer of a lacquer structure produced at elevated temperature onto an unheated band, the rapid lacquer cooling on the substrate ensures an increase in viscosity and thus fixing of the structures produced. Also of importance are the hardness of the rubber rollers employed and their contact pressure. Any structures desired can be produced both with surface-structured rollers and with smooth rollers. Structured rollers are known in various forms. Of these, particular mention should be made of engraved rollers. These can have recesses (pyramidal indents, hexagonal indents, etc.), line grids (hatched rollers), diagonal line grids or combinations thereof.
If a non-planar structure is desired, it can also be produced after the coating, or one that is already present can be subsequently changed. Thus, a structured surface can be smoothed using a distributor roller (for example a counter-rotating chrome roller). A longitudinally oriented line structure can be achieved using a structured roller or an attached coating bar (wire coating bar or grooved coating bar).
One lacquer structuring possibility that can be used is the viscosity dependence of the application temperature. By xe2x80x9cquenchingxe2x80x9d of a lacquer warmed to above room temperature during application on an unheated band, the application structure can be retained until curing. By preheating the support material, the lacquer flow can be improved and the surface structure levelled.
The dimensionally stable, two-dimensional support can be produced from a multiplicity of materials. Suitable are, for example, supports made from plastic film (in particular polyester films, especially polyethylene terephthalate films), but preferably supports made from a metal or a metal alloy. Of these, preference is in turn given to supports made from aluminium or an aluminium alloy. The front of the aluminium support is advantageously mechanically and/or electrochemically roughened and/or anodically oxidized and, if necessary, additionally hydrophilized (for example by treatment with polyvinylphosphonic acid). However, further layers between the support and the image layer are likewise possible, for example hydrophilizing layers or priming layers. The support may also be provided with a layer of a ceramic material (additive graining). The thickness of the support is generally from 0.1 to 1.0 mm, preferably from 0.2 to 0.6 mm.
It is possible for the support provided with the back coating to be rolled up again (xe2x80x9ccoil-to-coilxe2x80x9d process). Since the back is mechanically stable, it is virtually undamaged in the process, even in the interior of the roll, where the highest pressure prevails and the greatest forces act thereon.
Depending on the nature of their composition, the image layer may be sensitive to UV radiation, visible light and/or IR or heat radiation. The radiation-sensitive component in the image layer may, for example, be a diazonium salt, a combination of a photopolymerization initiator and a polymerizable monomer (in particular a monomer containing a polymerizable ethylenically unsaturated group), a combination of a compound which forms acid on irradiation, and a compound which can be cleaved by the photochemically generated acid. The image layer may additionally comprise IR dyes, carbon black and/or sensitizers.
Use is particularly frequently made in positive-working image layers of esters of a 1,2-naphthoquinone-2diazido-4- or -5-sulphonic acid and a compound containing at least one phenolic hydroxyl group. The last-mentioned compound preferably has at least 3 phenolic hydroxyl groups. Very particular preference is given for the esterification to compounds containing from 3 to 6 phenolic hydroxyl groups. Examples thereof are 2,3,4-trihydroxybenzophenone, 2,3,4-trihydroxy-3xe2x80x2-methyl-, -propyl- or -isopropylbenzophenone, 2,3,4,4xe2x80x2-tetra-hydroxybenzophenone, 2,3,4,2xe2x80x2,4xe2x80x2-pentahydroxybenzo-phenone, 2,3,4,2xe2x80x2,3xe2x80x2,4xe2x80x2-hexahydroxybenzophenone and 5,5xe2x80x2-diacyl-2,3,4,2xe2x80x2,3xe2x80x2,4xe2x80x2-hexahydroxydiphenylmethane. In general, not all the phenolic hydroxyl groups therein are esterified. The degree of esterification, based on all hydroxyl groups, is typically from 60 to 95%. Amides of 1,2-naphthoquinone-2-diazido-4- or -5-sulphonic acid are likewise suitable. Esterification components which can be used are also products of the condensation of pyrogallol and aldehydes or ketones and products of the condensation of alkylated phenol and formaldehyde. The content of radiation-sensitive compounds is from about 1 to 50% by weight, based on the total weight of the non-volatile constituents of the mixture. Image layers comprising naphthoquinonediazidosulphonic acid esters or -sulfonamides as radiation-sensitive component are particularly sensitive to UV and visible light.
Positive-working image layers which are insensitive to UV radiation and visible light, but can be imaged by IR or heat radiation are likewise known (EP-A 900 653). The layer comprises, as radiation-sensitive components, carbon black particles or a dye in disperse form which is sensitive in the IR region. IR radiation, in particular IR laser radiation, effects imagewise differentiation in the layer, enabling the irradiated areas to be removed by a developer.
It is also possible to use recording materials having a positive-working image layer which comprises a combination of a compound containing at least one Cxe2x80x94Oxe2x80x94C bond which can be broken by acid and a compound which forms a strong acid on exposure to actinic radiation. Layers of this type are known to the person skilled in the art and are described in large number, for example EP-A 717 317.
Besides a polymeric binder, photopolymerizable image layers usually comprise a free-radical-polymerizable component (monomer) and an initiator which is capable of initiating polymerization of the monomer on exposure to actinic radiation. The initiator is, for example, a combination of a photoreducible dye and a metallocene, in particularly a titanocene. The monomers frequently contain free-radical-polymerizable acrylate or methacrylate groups. The light sensitivity of such layers can be increased still further by employing monomers containing at least one photooxidizable group or additionally onium compounds, in particular iodonium or sulphonium salts. Photopolymerizable layers are impaired by atmospheric oxygen. They are therefore often protected by a cover layer which is relatively impermeable to oxygen, but which can be removed completely again by aqueous developers.
The image layer may also comprise silver halide as the radiation-sensitive component. It then includes a silver halide emulsion layer. Preference is given to image layers which work by the silver complex diffusion transfer reversal process (abbreviated to DTR process). It then consists of two or more part layers, as described in greater detail in EP-A 410 500, 423 399 or 883 027. The lowermost, i.e. closest to the support, is usually a receiving layer comprising silver nuclei. The nuclei initiate the development of the silver complexes that have diffused in, giving a silver image when a suitable developer acts thereon. The development nuclei are preferably produced by application of colloidal silver, gold, platinum, palladium or other metals. They may furthermore consist of heavy-metal sulphides or selenides, for example sulphides of antimony, bismuth, cadmium, cobalt, lead, nickel, palladium, platinum, silver or zinc. Palladium sulphide and the nickel/silver sulphide NiS.Ag2S described in U.S. Pat. No. 4,563,410 are particularly suitable. Also suitable are polyselenides or polysulphides of heavy metals. In addition, dyes or pigments may be present as antihalo agents, either as a constituent of the nucleus layer or in a separate layer. The type of dye or pigment depends on the region of the spectrum in which the silver halide emulsion layer is sensitive. The nucleus layer is very thin (generally less than 0.5 xcexcm); it normally contains no binder. As already described, the nucleus layer is not absolutely necessary. If no such layer is present, constituents of the metallic support take on the role of the development nuclei. Finally, it is also possible to arrange the image receiving layer or nucleus layer on a separate support. DTR materials of this type consisting of two elements are known in principle.
A thin, silver-free interlayer, for example a layer of pigment and a hydrophilic, film-forming polymer, for example polyvinyl alcohol or pullulane, is located above the receiving layer. The next is a silver halide emulsion layer. The silver halide is, for example, silver chloride, bromide, bromoiodide, chlorobromoiodide or a mixture thereof. The silver halide advantageously comprises more than 90% by weight, based on the total weight of the silver halides, of silver chloride. In addition, small amounts of silver chloroiodide and/or silver bromide are frequently also present. The silver halide particles in the emulsion layer normally have a mean size of from 0.05 to 1.0 xcexcm, preferably from 0.25 to 0.45 xcexcm. They can also be produced by the core of the particles having a different composition than the shell. Silver bromide is frequently located exclusively in the core. The binders used for this layer are generally hydrophilic colloids, preferably gelatine. The gelatine is advantageously not hardened. Instead of or in addition to the gelatine, it is also possible to employ other polymers, for example polyvinyl alcohol, polyvinylpyrrolidone, polyvinylimidazole, poly(meth)acrylamide, polyacrylic acid, cellulose or cellulose derivatives (particularly cellulose ethers, such as hydroxyalkyl- or carboxymethylcellulose), starch or alginates. Finally, the emulsion layer may also comprise dyes in order to adjust the spectral sensitivity of the silver halide layer and/or in order to prevent undesired light scattering. These are, for example, methine, cyanine or hemicyanine dyes. Finally, the silver halide layer may comprise conventional emulsion stabilizers, for example azaindenes, especially tetra- or pentaazaindenes. The azaindenes are preferably substituted by amino or hydroxyl groups. An example of a substituted azaindene of this type is 4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene. Other suitable stabilizers are quaternized benzothiazoles, benzotriazoles and heterocyclic mercapto compounds, for example mercapto-substituted tetrazoles and pyrimidines. An example of a tetrazole of this type is 1-[3-(2-sulphobenzoylamino)phenyl]-5-mercaptotetrazole.
In a preferred embodiment, a protective layer may also be located on the silver halide emulsion layer. It generally has a weight of from 0.50 to 1.75 g/m2 preferably from 0.60 to 1.20 g/m2 and advantageously consists of unhardened gelatine (a 10% strength by weight aqueous solution of the gelatine has a viscosity of preferably less than 20 mPaxc2x7s at 40xc2x0 C. and at pH 6). The cover layer may in turn comprise dyes and/or coloured pigments and/or matting agents. The matting agent here generally consists of particles having a mean diameter of from 0.2 to 10 xcexcm, preferably from 0.5 to 6.0 xcexcm.
Negative-working layers which are provided for imaging with UV or visible light in many cases comprise diazonium salt polycondensation products. These are, in particular, products of the condensation of aromatic diazonium salts. Condensation products of this type are known, inter alia, from DE-A 12 14 086 (=U.S. Pat. No. 3,235,384). They are generally prepared by condensation of a polycyclic aromatic diazonium compound, preferably of substituted or unsubstituted diphenylamine-4-diazonium salts with active carbonyl compounds, preferably formaldehyde, in a strongly acidic medium, preferably concentrated phosphoric acid.
The image layer may also be imaged by an electrophotographic principle. In this case, it usually comprises a photoconductive layer comprising an organic photo-conductor on an electrically conductive support.
In addition to the radiation-sensitive component, the image layer usually also comprises a polymeric, organic binder. Preference is given to phenolformaldehyde condensates, where the term xe2x80x9cphenolxe2x80x9d here is also taken to mean substituted phenols, such as resorcinol, cresol, xylenol, and the like. Besides or in addition to the formaldehyde, it is also possible to employ other aldehydes or also ketones as condensation partner. Also suitable are products of the reaction of diisocyanates with diols or diamines, in particular those containing acid groups. Mention should also be made of polymers containing units of vinylaromatic compounds, N-aryl-(meth)acrylamides or aryl (meth)acrylates, where these units in each case also contain one or more carboxyl group(s), phenolic hydroxyl groups, sulphamoyl or carbamoyl groups, as well as copolymers containing units of N-substituted maleimidobenzenesulfonamides.
If the recording material according to the invention is pigmented or matted on the front, the Bekk smoothness of the surface on this side is generally less than 600 s, preferably from 100 to 150 s.
The further processing (imagewise exposure or irradiation, development, etc.) for the recording materials according to the invention is carried out virtually in the same way as for recording materials without back coatings. Since the back coating is resistant to processing chemicals, it also prevents attack by the developer on the support. This is particularly important in the case of aluminium supports. These are attacked by alkaline developers, in particular by strongly alkaline developers, which increases the developer load and thus reduces its service life.
The following examples serve to illustrate the invention. pbw therein stands for part(s) by weight. Percentages are per cent by weight, unless otherwise stated.